Hitherto, various color developers for pressure-sensitive recording media (hereafter merely referred to "color developer") have been known, for example, (1) inorganic solid acids such as acid clay and apatalgide as described in U.S. Pat. No. 2,712,507, (2) semi-synthesized solid acids having a magnesium and/or aluminum component incorporated into acid treated clay minerals as described in JP-A-58-217389 (the term "JP-A" herein used means an unexamined and published Japanese patent application) (3) substituted phenols and diphenols as described in JP-B-40-9309 (the term "JP-B" herein used means an examined Japanese patent publication), (4)p-substituted-phenol/formaldehyde copolymer as described in JP-B-42-20144, and (5) metal salts of aromatic carboxylic acids as described in JP-B-49-10856 and JP-B-52-1327. In particular, novolak phenol resins and metal salts of substituted salicylic acids, which are called organic color developers in contrast to inorganic color developers such as active clay, have been widely put in practical use for pressure-sensitive recording media, as described in JP-B-42-20144 and JP-B-51-25174. These color developers are finely ground, dispersed generally in water as a medium, mixed with inorganic pigments, adhesives and the like, and coated on paper. The thus coated paper is used as a color developer sheet, as described in JP-B-48-16341 and JP-A-54-143322.
Color developer sheets are required to have the following properties: exhibiting good color developing ability unchanged even after long storage; being free from yellowing due to exposure to radiation (e.g., daylight); and providing color images which do not easily disappear or fade upon exposure to radiation, or in contact with water or plasticizers.
Color developers and color developer sheets which have been already proposed have both merits and demerits. For instance, inorganic solid acids are inexpensive but cause yellowing or deterioration in the color developing ability upon adsorption of gas or moisture in air. Substituted phenols have insufficient color developing ability and provide color images of poor density, and they are often used in the form of a copolymer with an aldehyde such as p-phenylphenol novolak resins. The p-phenylphenol novolak resins exhibit excellent color developing ability, but their coated sheets undergo yellowing by daylight exposure or during the storage (particularly due to nitrogen oxides in air) and the color images developed markedly fade.
Metal salts of aromatic carboxylic acids have good properties with respect to color developing ability, yellowing and fading. In order to remove crystallinity, however, those conventionally used have substituents (e.g., an alkyl group, a phenyl group, a cycloalkyl group, etc.) introduced onto the benzene ring, so that they are expensive and suffer many problems in coating on a sheet due to marked bubbling when dispersed in water.
Metal salts of substituted salicylic acids are normally non-crystalline solid. Since color developers are generally coated in the form of a dispersion in water, they are highly desired to have good workability at high concentration and good dispersion stability. However, when coarse particles of the above metal salt color developer are ground in water with a ball mill, a sand grinder or the like to a desired size suitable for coating, the resulting dispersion exhibits thixotropic properties and poor fluidity, so that it is difficult to handle the dispersion. For improving its fluidity, the color developer concentration must be lowered, or a large amount of dispersants should be added but in turn, causes remarkable bubbling. Alternatively, by dissolving the metal salt color developer in an organic solvent and then dispersing it in an aqueous solution containing a dispersant with vigorous stirring, an emulsion having good fluidity can be obtained even at high concentration. Since the dispersed particles are droplets containing the organic solvent, however, the dispersed droplets become large in size during long storage of the emulsion, and consequently the emulsified state tends to corrupt at the vicinity of vessel walls, deteriorating stability of the emulsion.